Abstract: Frequency discriminators are disclosed employing a crystal filter having a predetermined voltage versus frequency characteristic including a discriminator operating portion slightly below the filter lower cutoff frequency along which the voltage varies substantially linearly as a function of frequency. Detector circuitry rectifies the output voltage from the crystal filter, while voltage level shifting circuitry algebraically combines the output voltage from the detector with a dc voltage such that the algebraic sum of the dc voltage and the detector output voltage is zero when the instantaneous frequency of the input signal to the discriminator is at the discriminator center frequency. The discriminator is constructed without the use of any crystal resonator having a series resonant frequency within the discriminator passband; hence spurious series resonances occur outside of the discriminator passband, thereby facilitating wideband operation.